Vacuum systems often comprise a main vacuum pump which is driven by a drive motor and associated with various sensors, valves and other peripheral devices. The main vacuum pump may also be associated with a vacuum roughing pump and a secondary pump for specific gases such as water vapor. Cryopumps and turbomolecular pumps, for example, generally include temperature and pressure sensors and purge and roughing valves. A turbomolecular pump may also be associated with a cryopump such as a single stage cryogenic water pump. The cryogenic water pump would also have associated sensors and control valves.
Cryogenic vacuum pumps, or cryopumps, currently available generally follow a common design concept. A low temperature array, usually operating in the range of 4 to 25K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 60 to 130K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and a work chamber to be evacuated.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In systems cooled by closed cycle coolers, the cooler is typically a two-stage refrigerator having a cold finger which extends through the rear or side of the radiation shield. High pressure helium refrigerant is generally delivered to the cryocooler through high pressure lines from a compressor assembly. Electrical power to a displacer drive motor in the cooler is usually also delivered through the compressor.
The cold end of the second, coldest stage of the cryocooler is at the tip of the cold finger. The primary pumping surface, or cryopanel, is connected to a heat sink at the coldest end of the second stage of the cold finger. This cryopanel may be a simple metal plate or cup or an array of metal baffles arranged around and connected to the second-stage heat sink. This second-stage cryopanel also supports the low temperature adsorbent.
The radiation shield is connected to a heat sink, or heat station, at the coldest end of the first stage of the refrigerator. The shield surrounds the second-stage cryopanel in such a way as to protect it from radiant heat. The frontal array is cooled by the first-stage heat sink through the side shield or, as disclosed in U.S. Pat. No. 4,356,701, through thermal struts.
After several days or weeks of use, the gases which have condensed onto the cryopanels, and in particular the gases which are adsorbed, begin to saturate the cryopump. A regeneration procedure must then be followed to warm the cryopump and thus release the gases and remove the gases from the system. As the gases evaporate, the pressure in the cryopump increases, and the gases are exhausted through a relief valve. During regeneration, the cryopump is often purged with warm nitrogen gas. The nitrogen gas hastens warming of the cryopanels and also serves to flush water and other vapors from the cryopump. By directing the nitrogen into the system close to the second-stage array, the nitrogen gas which flows outward to the exhaust port minimizes the movement of water vapor from the first array back to the second-stage array. Nitrogen is the usual purge gas because it is inert and is available free of water vapor. It is usually delivered from a nitrogen storage bottle through a fluid line and a purge valve coupled to the cryopump.
After the cryopump is purged, it must be rough pumped to produce a vacuum about the cryopumping surfaces and cold finger to reduce heat transfer by gas conduction and thus enable the cryocooler to cool to normal operating temperatures. The rough pump is generally a mechanical pump coupled through a fluid line to a roughing valve mounted to the cryopump.
Control of the regeneration process is facilitated by temperature gauges coupled to the cold finger heat stations. Thermocouple pressure gauges have also been used with cryopumps but have generally not been recommended because of a potential of igniting gases released in the cryopump by a spark from the current-carrying thermocouple. The temperature and/or pressure sensors mounted to the pump are coupled through electrical leads to temperature and/or pressure indicators.
Although regeneration may be controlled by manually turning the cryocooler off and on and manually controlling the purge and roughing valves, a separate regeneration controller is used in more sophisticated systems. Leads from the controller are coupled to each of the sensors, the cryocooler motor and the valves to be actuated.
Another form of vacuum pump used in high vacuum systems, such as semiconductor processing systems, is the turbomolecular pump. A turbomolecular pump comprises a high speed turbine which drives the gas molecules. Since the turbomolecular pump operates most efficiently in the molecular flow region, the gas molecules which are driven through the pump are removed by a roughing vacuum pump which maintains a vacuum in the order of 10.sup.-3 torr at the foreline, or exhaust, of the turbomolecular pump.
Because the gas as being pumped by the turbomolecular pump may be extremely corrosive or hazardous in other ways, it is often diluted by a purge gas in the foreline region of the pump. To that end, a purge valve is coupled to the pump to introduce purge gas from an inert gas supply. The purge gas is typically introduced into the motor/bearing region.
During shutdown of the pump, gas is typically introduced about the turbine blades through a separate vent valve. The vent gas prevents back streaming of hydrocarbons from the bearing lubricants in the foreline and assists in slowing of the pump by introducing a fluid drag.
To allow the turbomolecular pump to operate more effectively, some systems use a heater blanket about the housing to warm the blades and housing during operation and to thus evaporate any condensed gases. During continued operation, cooling water is circulated through the pump to prevent overheating of the bearings. Typical systems include a sensor for sensing bearing temperature in order to provide a warning with overheating.
A rack mounted control box is generally used to convert power from a standard electrical outlet to that required by the pump drive motor. The motor driving the turbine is typically a DC brushless motor driven through a speed control feedback loop or an AC synchronous motor. More sophisticated controllers may be connected to the various valves of the system to open and close those valves according to some user programmable sequence. Leads from the controller are coupled to the pump drive motor, the temperature sensor and each valve to be actuated.